Elevation estimation method and radar apparatus using it

ABSTRACT

The invention relates to an elevation estimation method and a radar apparatus of the stacked beam type using it. For the accurate determination of the elevation of a object, use is made of interpolation on the basis of object strengths, measured in the different beams. Thus, an object of this invention is a method for estimating an object&#39;s elevation comprising: receptions and associated receiving processing of signal reflected by the object, a beam transformation of said processed received signals into a number of receiving antenna beams which are at least substantially identical in azimuth direction and distributed in elevation direction, an interpolation of the receiving antenna beams, which are provided with a mutual overlap, such as the object&#39;s elevation is deduced by standardization of set of strengths measured along the elevation direction distribution and comparison to pre-defined sets of strengths which have been associated with elevations, the comparison determining a best fitting set. In order to make interpolation also possible for low-flying objects, at least one beam having a negative elevation is provided.

The invention relates to an elevation estimation method and a radarapparatus of the stacked beam type using it. In particular, it relatesto radar apparatus comprising a transmitting antenna, transmittingmeans, and a receiving antennas system, each receiving antenna being fedby associated receiving means, said radar apparatus being suitable forthe emission of radar transmit signals and the subsequent reception ofreflected radar transmit signals, said reflected radar transmit signalsfeeding a beam former after reception by the individual receivingantennas and processing by the associated receiving means, in order toobtain a number of receiving antenna beams which are at leastsubstantially identical in azimuth direction and distributed inelevation direction.

A radar apparatus of this type is known from EP-A-0110260. The advantageof a radar apparatus, which forms several beams, at least in elevationdirection, is that, from an observed object, the elevation direction isknown and also, combined with the usually available range information,the object height. Herewith is the choice of the beam width veryimportant. Firstly, certainty is desired that an object is actuallybeing observed, so that some overlap of the beams is unavoidable.Secondly, an object is minded to be observed in not more than one beamto avoid overloading the video processor connected to the radarapparatus.

This invention solves the above-mentioned drawbacks by interpolatingreceiving antenna beams, which are provided with a mutual overlap.

An object of this invention is a method for estimating an object'selevation comprising:

-   Receptions and associated receiving processing of signal reflected    by the object,-   A beam transformation of said processed received signals into a    number of receiving antenna beams which are at least substantially    identical in azimuth direction and distributed in elevation    direction,-   An interpolation of the receiving antenna beams, which are provided    with a mutual overlap, such as the object's elevation is deduced by    standardisation of set of strengths measured along the elevation    direction distribution and comparison to pre-defined sets of    strengths which have been associated with elevations, the comparison    determining a best fitting set.

Interpolation of the object information originating from the differentreceiving antenna beams can take place in a variety of ways. Anadvantageous method is characterised in that the interpolation stepcomprises the elevation determination on the basis of the strength ofthe reflected radar transmit signals present at the output of thedifferent receiving antenna beams. By first determining those strengths,interpolation can take place using a number of scalars, for which littlehardware is required.

Objects at low elevation, which generally are the most relevant ones, inparticular in known radar apparatuses of this type are present in onereceiving antenna beam only, which would make interpolation impossible.A further advantageous embodiment of the inventive elevation estimationmethod is that the beam transformation generates at least one beamhaving a negative elevation direction. In this way, for a low-elevationobject nevertheless two strengths are established, allowing a form ofinterpolation.

A very advantageous realisation of the elevation estimation method isthat the beam transformation generates at least two beams having anegative elevation direction, the transformation preferably generates atleast four beams that are equidistant in elevation direction, such thata first beam has a first positive elevation direction, that a secondbeam has a second, smaller positive elevation direction, that a thirdbeam has a negative elevation direction whose absolute magnitudecorresponds to that of the second elevation direction, and that a fourthbeam has a negative elevation direction whose absolute magnitudecorresponds to that of the first elevation direction. Thus, in the caseof a low-elevation object, four strengths are obtained, enablingexcellent interpolation.

According to a further very advantageous embodiment, the elevationestimation method according to the invention is characterised in thatthe interpolation step processes the strengths of reflected radartransmit signals from a object, obtained from the four beams incombination, such as the processing eliminates measuring errors due tolobbing and mirror effect. An advantageous implementation of thisembodiment is characterised in that the interpolation determines aquotient of a strength in the fourth beam minus a strength in the firstbeam to a strength in the third beam minus a strength in the secondbeam, and reads the elevation of the object within a given table usingthe quotient.

Another object is a radar apparatus of the stacked beam type beingsuitable for the emission of radar transmit signals and the subsequentreception of reflected radar transmit signals, using the elevationestimation method according to any of the preceding claims, said radarapparatus comprising:

-   A transmitting antenna,-   Transmitting means,-   A receiving antennas system-   Receiving means, each receiving means being fed by associated    receiving antennas, and-   A beam former being fed with said reflected radar transmit signals    feeding after reception and processing by the individual receiving    antennas and their associated receiving means, said beam former    implementing the beam transformation such as to obtain a number of    receiving antenna beams which are at least substantially identical    in azimuth direction and distributed in elevation direction,-   Interpolation means connected to the beam former, said receiving    antennas, receiving means and/or said beam former being arranged    such as the beam former provides receiving antenna beams with a    mutual overlap to the interpolation means, which determines the    object's elevation.

An advantageous implementation is characterised in that theinterpolation means has been equipped with standardisation means forstandardising the object strengths, and with comparison means forcomparing the standardised object strengths with a system of previouslydetermined foursomes of standardised object strengths, for determining abest fitting foursome and deriving from it the object elevation.

Further features and advantages of the invention will be apparent fromthe following description of examples of embodiments of the inventionwith reference to the drawing, which shows details essential to theinvention, and from the claims. The individual details may be realisedin an embodiment of the invention either severally or jointly in anycombination.

FIG. 1, a block diagram of the elevation estimation method according tothe invention,

FIG. 2, a block diagram of the radar apparatus according to theinvention,

FIG. 3, a radar apparatus comprising eleven receiving antenna beams,

FIG. 4, a radar apparatus comprising twelve receiving antenna beams,

FIG. 5, a radar apparatus comprising thirteen receiving antenna beams.

FIG. 1 shows a block diagram of the elevation estimation methodaccording to the invention comprising a reception step [S3] followed bya receiving processing step [S4] for receiving and further process thesignals reflected by the object whose elevation will be estimated. Thus,are obtained several processed received reflected signals. These signalsare then transformed during the beam transformation step [S5] inreceiving antenna beams with a mutual overlap. The interpolation step[S6] determines from said receiving antenna beams the object'selevation.

FIG. 2 shows a block diagram of a radar apparatus according to theinvention, comprising transmitting means 1, a transmitting antenna 2,receiving antennas 3 a, . . . , 3 p (for example, sixteen receivingantennas), associated receiving means 4 a, . . . , 4 p, a beam former 5and interpolation means 6. The transmitting means 1 feed a transmittingantenna 2 with radar transmit signals. The sixteen receiving antennas 3a, . . . , 3 p receive the radar transmit signals reflected by apotential object. The reflected radar transmit signals are passed on,via receiving means 4 a, . . . , 4 p, to a beam former 5. The beamformer 5 generates from these reflected signals beams of differentelevations, for example, eleven beams as shown in FIG. 3. The outputsignals of beam former 5 are subsequently applied to interpolation means6, which accurately determines the elevation of the observed object.

Transmitting antenna 2 and receiving antennas 3 a, . . . , 3 p aremechanically connected such that their azimuth directions are identical.The individual antennas may comprise a linear array of dipole antennasand a feeder network that is constructed from foam stripline to keep theweight down.

Transmitting antenna 2 and receiving antennas 3 a, . . . , 3 p may behoused in a common antenna array 7. Antenna array 7 could have beenarranged rotatably, such that the radar apparatus is able to provide a3D representation of the environment, which means that at least therange, the azimuth and the elevation of an object can be determined.

It is possible for transmitting antenna 2 and receiving antennas 3 a, .. . , 3 p to be combined, with at least one receiving antenna 3 i beingequipped with Transmit/Receive means, such that also radar transmitsignals can make use of the at least one receiving antenna.

Receiving means 4 a, . . . , 4 p may be of a type that is well known inthe radar discipline, preferably of the heterodyne type and providedwith a limiter, a low-noise amplifier, and possibly a pulse compressionnetwork. Additionally, receiving means may be equipped with cluttersuppression means, in particular on the basis of the Doppler effect, forexample a canceller or a DFT (Digital Fourier Transform) processor.

The output signals of receiving means 4 a, . . . , 4 p may be of theanalogue quadrature type if beam former 5 is a Butler matrix, and of thedigital quadrature type if the beam former comprises a DFT. For eachemitted radar signal, the outputs of beam former 5 generate a signal,which can be regarded as split up into range quants. If in a receivingantenna beam a object is observed, the relevant output signal in a rangequant corresponding with the distance from the object to the radarapparatus will considerably exceed an always present noise level, whichcan easily be detected with the aid of a threshold circuit commonlyknown in the radar discipline. If, in a specific range quant an objectis detected in this manner, the receiving antenna beams adjacent to thatrange quant are also examined if there, too, detection has been made. Byensuring that neighbouring receiving antenna beams overlap, this willalways be the case, provided that the object is sufficiently strong, or,in other words, provided that the signal-to-noise ratio is sufficient.

Interpolation means 6 receives the object strengths per range quant forthe different receiving antenna beams, and on their basis estimates thecurrent elevation of the object. For this, a linear interpolation on thebasis of the object strengths may be used, but better results areattained by standardising the object strengths and subsequentlycomparing them to a collection of object strengths, calculated for thedifferent elevations.

Interpolation means 6, which preferably operate digitally, may comprisea number of DSPs. If beam generator 5 is arranged as a Butler matrix,the interpolation means will have its inputs provided with A/Dconverters, which per range quant digitalise the output signals of beamformer 5.

If an object is close to the surface of the earth, it may happen thatonly the lowest beam generates detection, so that an interpolation isnot possible. This is especially detrimental because objects of interestpreferably are close to the earth's surface. FIG. 4 shows the beams of afirst embodiment of the radar apparatus, which avoid this drawback byadding a receiving antenna beam 41 having a negative elevation direction42. This antenna beam will, as it is well known in the radar discipline,be subject to a mirror reflection from the earth's surface, but willnevertheless generate an additional object strength. This objectstrength and the object strength generated in the lowest receivingantenna beam of FIG. 3 are dependent on the unknown size and elevationof the object, and on the known frequency of the radar transmit signals,the height of the antennas above the earth's surface and on the knowndistance from the object to the radar apparatus. It is then possible tocalculate, for each frequency, pairs of object strengths, and to comparethese with a pair of measured object strengths that were adopted asstandards. From this, per emitted radar transmit signal the elevation ofthe object can be estimated. Through averaging the estimates for anumber of successively emitted radar transmit signals, an accurateestimation of the object's elevation is acquired.

A further improvement in determining the elevation of an object can beachieved by adding two negative-elevation beams 51, 52 as shown in FIG.5. By this means, for a object that is close to the surface of theearth, some four object strengths will be generated, with the strengthsagain depending on the size and elevation of the object and on the knownfrequency of the radar transmit signals, the height of the antennasabove the surface of the earth, and on the known distance from theobject to the radar apparatus. It is then possible to calculate for eachfrequency a system of foursomes of standardised strengths, and tocompare them with a measured object-strength foursome that was adoptedas a standard, a best fit, for example on the basis of a least squaresmethod, accurately yielding the elevation of the object. Alternatively,this system of foursomes could be measured in a series of test flightswith a object of known radar cross-section, with the test flightsneeding to be flown at different altitudes, and measurements needing tobe made on the operationally significant frequencies.

Comparing foursomes of standardised strengths with a system of foursomesof standardised strengths requires much computing time. It has beenfound to be possible instead of this to compare a single number with atable, which table, however, may be determined per operationallysignificant frequency. If the four relevant beams are identified,starting from the topmost one, as beam one, beam two, beam three andbeam four, then the number is found by subtracting the object strengthin beam four from the object strength in beam one, by subtracting theobject strength in beam three from the object strength in beam two,subsequently determining the quotient from the two differences. Thetable may again be determined per frequency with reference totheoretical considerations, or through executing a number of flightswith a object of known radar cross-section, measurements needing to betaken at different frequencies.

In establishing the elevation of a object in this manner, it may happenthat the quotient cannot be determined. It is then necessary for themeasurement to be executed once more, choosing a different frequency forthe radar transmit signals. In more general terms, the determination ofa object's elevation will not be based on a single measurement, but onrepeatedly measuring at several different frequencies, after which somefiltering is still possible.

1. A method for estimating an object's elevation comprising the stepsof: receptions and associated receiving processing of a signal reflectedby the object, a beam transformation of said processed received signalsinto a number of receiving antenna beams which are at leastsubstantially identical in azimuth direction and distributed inelevation direction, measuring a set of object strengths by a set ofantenna beams substantially identical in azimuth, these beams beingprovided with a mutual overlap, and comparing with the measured objectstrengths sets of previously pre-defined standardized strengths, fordetermining a best fitting set and deriving the elevation of the objectfrom the elevation associated with the best fitting set, and generatingthe beam transformation into two beams having a negative elevationdirection.
 2. The elevation estimation method according to claim 1,wherein said mutual overlap is such as an object is always observed inat least two receiving antenna beams.
 3. The elevation estimation methodaccording to claim 1, wherein the interpolation step determines theelevation on the basis of the strength of the reflected radar transmitsignals using the different receiving antenna beams.
 4. The elevationestimation method according to claim 1, wherein the beam transformationgenerates at least four beams that are equidistant in elevationdirection
 5. The elevation estimation method according to claim 1,wherein: the first beam has a first positive elevation direction, thesecond beam has a second, smaller positive elevation direction, thethird beam has a negative elevation direction whose magnitude is equalto that of the second elevation direction, and the fourth beam has anegative elevation direction whose magnitude is equal to that of thefirst elevation direction
 6. The elevation estimation method accordingclaim 6, wherein the interpolation step processes the strengths ofreflected radar transmit signals from a object, obtained from the fourbeams in combination, such as the strength processing eliminatesmeasuring errors due to lobbing and mirror effect.
 7. The elevationestimation method according to claim 1, wherein the interpolation stepdetermines a quotient of a strength in the fourth beam minus a strengthin the first beam to a strength in the third beam minus a strength inthe second beam, and reads the elevation of the object within a giventable using the quotient.
 8. A radar apparatus of the stacked beam typebeing suitable for the emission of radar transmit signals and thesubsequent reception of reflected radar transmit signals, using theelevation estimation method according to any of the preceding claims,said radar apparatus comprising: a transmitting antenna, transmittingmeans, a receiving antennas system (3 a, . . . , 3 p, receiving means (4a, . . . , 4 p), each receiving means (4 a, . . . , 4 p) being fed byassociated receiving antennas (3 a, . . . , 3 p), and a beam formerbeing fed with said reflected radar transmit signals after reception andprocessing by the individual receiving antennas (3 a, . . . , 3 p) andtheir associated receiving means (4 a, . . . , 4 p), said beam formerimplementing the beam transformation such as to obtain a number ofreceiving antenna beams which are at least substantially identical inazimuth direction and distributed in elevation direction, wherein saidradar apparatus further comprises interpolation means connected to thebeam former, said receiving antennas (3 a, . . . , 3 p), receiving means(4 a, . . . , 4 p) and/or said beam former being arranged such as thebeam former provides receiving antenna beams with a mutual overlap tothe interpolation means, which determines the object's elevation.
 9. Theradar apparatus according to claim 8, wherein the interpolation meanshas been equipped with standardization means for standardizing thestrengths, and with comparison means for comparing the standardizedstrengths with a system of previously determined foursomes ofstandardized strengths, for determining a best fitting foursome andderiving from it the elevation of the object.
 10. The radar apparatusaccording to claim 9, wherein the interpolation means has been providedwith a table enabling, using said quotient, the elevation of the objectto be read.
 11. The elevation estimation method according to claim 2,wherein the interpolation step determines the elevation on the basis ofthe strength of the reflected radar transmit signals using the differentreceiving antenna beams.
 12. The elevation estimation method accordingto claim 2, wherein the beam transformation generates at least fourbeams that are equidistant in elevation direction
 13. The elevationestimation method according to claim 3, wherein the beam transformationgenerates at least four beams that are equidistant in elevationdirection
 14. The elevation estimation method according to claim 2,wherein: the first beam has a first positive elevation direction, thesecond beam has a second, smaller positive elevation direction, thethird beam has a negative elevation direction whose magnitude is equalto that of the second elevation direction, and the fourth beam has anegative elevation direction whose magnitude is equal to that of thefirst elevation direction.
 15. The elevation estimation method accordingto claim 3, wherein: the first beam has a first positive elevationdirection, the second beam has a second, smaller positive elevationdirection, the third beam has a negative elevation direction whosemagnitude is equal to that of the second elevation direction, and thefourth beam has a negative elevation direction whose magnitude is equalto that of the first elevation direction.
 16. The elevation estimationmethod according to claim 4, wherein: the first beam has a firstpositive elevation direction, the second beam has a second, smallerpositive elevation direction, the third beam has a negative elevationdirection whose magnitude is equal to that of the second elevationdirection, and the fourth beam has a negative elevation direction whosemagnitude is equal to that of the first elevation direction.
 17. Theelevation estimation method according claim 14, wherein theinterpolation step processes the strengths of reflected radar transmitsignals from a object, obtained from the four beams in combination, suchas the strength processing eliminates measuring errors due to lobbingand mirror effect.